EP1816818A1 - Estmation de la synchronisation de symbole dans un système OFDMA - Google Patents

Estmation de la synchronisation de symbole dans un système OFDMA Download PDF

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Publication number
EP1816818A1
EP1816818A1 EP06300097A EP06300097A EP1816818A1 EP 1816818 A1 EP1816818 A1 EP 1816818A1 EP 06300097 A EP06300097 A EP 06300097A EP 06300097 A EP06300097 A EP 06300097A EP 1816818 A1 EP1816818 A1 EP 1816818A1
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EP
European Patent Office
Prior art keywords
pilot
sub
sub carriers
time shift
carriers
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP06300097A
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German (de)
English (en)
Inventor
Hardy Halbauer
Dr. Erhard Sailer
Jürgen Otterbach
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Alcatel Lucent SAS
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Alcatel Lucent SAS
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Filing date
Publication date
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Priority to EP06300097A priority Critical patent/EP1816818A1/fr
Priority to KR1020060116759A priority patent/KR20070079294A/ko
Priority to US11/637,049 priority patent/US20070177684A1/en
Priority to CNA2006101687929A priority patent/CN101014030A/zh
Publication of EP1816818A1 publication Critical patent/EP1816818A1/fr
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2673—Details of algorithms characterised by synchronisation parameters
    • H04L27/2675—Pilot or known symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2662—Symbol synchronisation
    • H04L27/2665—Fine synchronisation, e.g. by positioning the FFT window
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/02—Speed or phase control by the received code signals, the signals containing no special synchronisation information
    • H04L7/033—Speed or phase control by the received code signals, the signals containing no special synchronisation information using the transitions of the received signal to control the phase of the synchronising-signal-generating means, e.g. using a phase-locked loop
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/04—Speed or phase control by synchronisation signals

Definitions

  • the invention relates to Orthogonal Frequency Division Multiple Access (OFDMA) burst transmission systems using multiple non-adjacent groups of sub carriers per OFDMA subscriber station (SS) with pilot signals within each sub carrier group for assisting time synchronization and channel estimation. More precisely the invention relates to a method for detecting symbol timing in such an OFDMA system.
  • OFDMA Orthogonal Frequency Division Multiple Access
  • the uplink of the currently evolving WiMAX (Worldwide Interoperability for Microwave Access) systems according to the IEEE 802.16e standard or the Chinese WIBRO (Wireless Broadband) standard are such systems.
  • IEEE 802.16e defines an OFDMA burst transmission physical layer using multiple non-adjacent groups 1 of sub carriers 2 per SS with pilot sub carriers 4 within each sub carrier group 1 (Fig. 2). Those non-adjacent groups are also called tiles. Six tiles 1 form a sub-channel, wherein each sub-channel is assigned to a SS. Also multiple sub-channels can be assigned to the same SS. Each tile 1 comprises four adjacent sub carriers 2 and three time symbols 3. Each tile 1 comprises four pilot sub carriers 4. The remaining sub carriers 2 in each sub carrier group 1 are data sub carriers 5.
  • the sub carriers 2, i.e. the data sub carriers 5 and the pilot sub carriers 4, which form a sub-channel, are spread over the frequency band, wherein sub carriers 2 assigned to a tile 1 always are adjacent sub carriers 2.
  • the sub carriers 2 having different frequencies and belonging to the same time symbol 3 form a row R, wherein succeeding sub carriers 2, i.e. sub carriers 2 belonging to different time symbols 3, having the same frequency form a column C.
  • IFFT inverse Fast Fourier Transformation
  • a Fast Fourier Transformation has to be performed at receiver side.
  • FFT Fast Fourier Transformation
  • perfect symbol timing i.e. a perfect synchronization between beginning and ending of the sampling per time symbol at the receiver side and the beginning and ending of the time symbols in the received OFDMA signal has to be performed by a timing estimation and timing tracking algorithm or procedure.
  • the FFT is performed in order to obtain the values magnitude and phase of the received, analog symbols of each sub carrier independently from the frequency of the particular sub carrier.
  • the values magnitude and phase - in the following just called values - can be displayed as real and imaginary part in the complex number plane in a way that the magnitude is displayed as the distance from the origin of the complex number plane and the phase is displayed as the angle versus positive real axis. In the following it is not differentiated between the sub carriers and the analog symbols of the sub carriers, i.e. the values of the sub carriers, if not mentioned explicit by using the term value.
  • OFDMA sub carriers usually have a cyclic extension, the so called cyclic prefix (CP), which allows FFT and demodulation even in case of non-ideal symbol timing adjustment.
  • CP cyclic prefix
  • this CP length is defined to be 1/8 of the OFDMA time symbol. This means that the number of samples per time symbol is larger than the FFT window size. Due to this, the signal can be correctly transformed and demodulated using FFT, as long as the symbol timing error is less than 1/8 of the time symbol duration.
  • Symbol timing errors within the CP lead to a phase rotation of the resulting values of the sub carriers.
  • Symbol timing errors larger than CP cause signal distortion due to inter-carrier interference. Ideal symbol timing is achieved, if the FFT uses samples between the end of the CP and the end of the time symbol.
  • symbol timing has to be derived from the received data sub carriers and pilot sub carriers available in the OFDMA signal.
  • BS Base Station
  • a precise correction of the time shift at the Base Station is mandatory.
  • a BS receives signals from several SSs that are all supposed to be within a timing tolerance, an exact synchronization of the BS with the SSs is not possible. Therefore the BS will have to correct the data sub carriers of the individual SSs on a per SS basis.
  • the symbol timing variation has to be monitored, to allow correction of the SS symbol timing, i.e. the individual ranging, before the allowed tolerances are violated.
  • An object of the invention is to find a remedy for this problem.
  • the object of the invention is met by the proposed method for detecting symbol timing in an OFDMA system, wherein a sub-channel comprises several groups of sub carriers spread over the frequency band, each group of sub carriers comprising at least one pilot carrier, said method comprising the steps of:
  • the correction is performed per sub-channel, and preferably per SS if more than one sub-channel is assigned to a SS, i.e. all pilot sub carriers belonging to the same sub-channel and preferably all pilot sub carriers belonging to the same SS are considered when determining the time shift showing the minimum standard deviation.
  • the standard deviation from the mean pilot sub carrier is calculated among the time shifted pilot sub carriers of the sub carrier groups of each sub-channel for each individual time shift of the set of time shifts.
  • the set of time shifts comprises different time shifts in constant intervals. The size of the intervals directly influences the exactness of the determined time shift showing the minimum standard deviation among the corrected pilots.
  • the time shift showing the minimum standard deviation to the mean pilot sub carrier is applied to the data sub carriers of the SS. Applying this time shift on the sub carriers, particularly on the data sub carriers within this sub-channel, preferably takes place by calculating individual correction factors for all sub carriers, wherein the individual correction factors are directly depending on the sub carrier frequency and the determined time shift showing the minimum standard deviation.
  • Said method according to the invention has the advantage over the state of the art, that it provides a higher performance in presence of multi-path effects. This is because according to the invention an average time shift for the whole sub-channel or for all sub-channels belonging to the same SS is determined. Such an average time shift also averages sub carrier specific multi-path effects such as frequency depending delay and attenuation.
  • a channel estimate of a pilot sub carrier is the quotient of the received values of the sub carrier to the known value of the pilot sub carrier at the transmitter. Doing so is of particular interest, if a modulation of the pilot sub carriers with randomly selected values '+1' and'-1' is performed according to IEEE802.16e standard. By regarding the channel estimates, the modulation is filtered. Again, if more than one sub-channel is assigned to a SS, all pilot sub carrier estimates are considered for determining the mean pilot channel estimate.
  • correcting the received pilot sub carriers or the calculated pilot sub carrier channel estimates with a set of different time shifts, calculating the standard deviation of the corrected pilot carriers or the calculated pilot sub carrier channel estimates for each time shift, selecting the time shift showing the minimum standard deviation, and applying said time shift to the received data sub carriers is performed per SS instead of per channel.
  • all sub-channels assigned to a particular subscriber station are considered, instead of an individual consideration per sub-channel. Doing so, it is achieved to use the pilot carriers of all sub-channels belonging to the same SS to derive the actual symbol timing. This increases reliability of the determined time shift.
  • the set of different time shifts used to correct the pilot signals lies within an expected tolerance.
  • the tolerance within an OFDMA system is defined as a fraction of the duration of a time symbol, e.g. 1/64 of the time symbol duration.
  • the set of time shifts comprises time shifts from ⁇ 1/64 of the symbol duration applied on the receiving time of the symbols to determine the actual symbol timing. Doing so, time shifts outside the tolerance are not considered and the complexity to calculate the standard deviation is reduced.
  • a FFT is performed in order to transfer and to treat the pilot sub carriers and preferably also the data sub carriers in the complex number plane before calculating the standard deviation of the corrected pilot sub carriers for each time shift.
  • treating the sub carriers comprises e.g. the determination of the mean pilot as well as applying the set of time shifts to the pilot sub carriers.
  • applying said time shift to the received data sub carriers is either done by shifting the FFT window to the appropriate position given by the selected time shift showing the minimum standard deviation, or the FFT output is corrected with the selected time shift showing the minimum standard deviation.
  • first of all the modulation of the pilot sub carriers is removed, e.g. by multiplying the pilot sub carriers with their nominal values +1 or -1, with which they are modulated e.g. in the IEEE802.16e standard, before the mean pilot is determined and before the pilot sub carriers are corrected with the set of different time shifts.
  • the pilot sub carriers originally having the values (1,0i) in the complex number plane, i.e. a magnitude of 1 1. Multiplying the received pilot sub carriers with ( ⁇ 1,0i) according to ⁇ the known random sequence ensures a positive magnitude without changing the phase of the received pilot sub carriers.
  • the resulting values of the pilot sub carriers are identical with the pilot sub carrier channel estimates. Therefore, instead of the pilot sub carrier channel estimates, also the values of the received pilot sub carriers can be used if a modulation according to IEEE 802.16e standard is performed on the transmitter side.
  • a fine correction for the symbol timing is performed in order to correct deviations caused by sample inaccuracy in the order of a fraction of the sampling interval.
  • the fine correction is obtained using the slope of the linear regression of the phase angles of the pilot sub carriers vs. the sub carrier frequency, after the correction with the time shift showing the minimum standard deviation and having only sample accuracy is performed in a previous step.
  • the coarse time corrected pilot sub carriers are rotated around the origin with the inverse phase angle of the mean pilot, or the mean pilot channel estimate if the channel estimates are considered, in order to have their mean value on the positive real axis before determining the slope of the linear regression of the phase angles vs. the sub carrier frequency.
  • the mean value is the statistical average value of all pilot sub carriers present doing mean or so called average calculation.
  • the mean value is identic to the mean pilot. Doing so, the advantage is achieved that common lookup tables and tools can be used to determine the slope of the phase signal vs. the sub carrier frequency without the risk of an ambiguity at 180°.
  • the slope of the imaginary parts of the pilot sub carriers vs. frequency is used instead of the slope of the phase angles.
  • fine correction takes place by determining and applying said fine correction after determining and applying the time shift showing the minimum standard deviation to the received data sub carriers.
  • Applying said fine correction preferably takes place by either shifting the FFT window to the appropriate position given by the selected time shift showing the minimum standard deviation plus the determined fine correction, or by correcting the FFT output with the selected time shift showing the minimum standard deviation plus the determined fine correction.
  • the symbol timing is monitored and tracked on the receiver side in order to detect, if the symbol timing exceeds a given tolerance, and in case of exceeding a given tolerance, symbol timing, i.e. ranging is adjusted on the transmitter side.
  • symbol timing i.e. ranging
  • the symbol timing has to be monitored and tracked on the receiver side in order to adjust the ranging on the transmitter side, if given tolerances are exceeded. So the actual symbol timing has to be detected with an accuracy higher than the adjustment steps and the given tolerance for operation.
  • ranging as defined e.g. in the IEEE 802.16e standard describes the adaption of symbol timing to the actual distance between a BS and a SS. If one of both moves, ranging has to be adapted continuously or step by step in intervals.
  • Another aspect of the invention concerns an OFDMA base station to be used to execute the method mentioned above, said OFDMA base station comprising:
  • a preferred embodiment of said OFDMA base station comprises means to perform a FFT before determining the mean pilot and correcting the received pilot carriers with the set of different time shifts.
  • said method mentioned above is performed by a computer program product stored on a computer usable medium comprising computer readable program means for causing a computer to perform the method mentioned above, when said computer program product is executed on a computer.
  • FIG. 1 showing a schematic flowchart of a method according to the invention
  • FIG. 2 showing a schematic view of the structure of an OFDMA sub-channel
  • Fig. 3 showing a diagram showing schematically the locations of pilot sub carriers in the complex plane in presence of multi-path effects and ideal timing
  • Fig. 4 showing a diagram showing schematically the locations of pilot sub carriers in the complex plane in presence of multi-path effects and non-ideal timing
  • Fig. 5 showing a diagram showing the standard deviation of pilot sub carrier values from the mean pilot for different time shifts
  • Fig. 6 showing a schematic view of the structure of a group of sub carriers, i.e. of a tile.
  • FIG. 1 A method according to the invention is executed in the following way (Fig. 1):
  • a FFT is performed in order to get the values magnitude and phase of the received sub carriers, i.e. of the pilot sub carriers and the data sub carriers independently from the frequency of the particular sub carrier.
  • the values can be displayed in the complex number plane in a way that the magnitude is displayed as the distance from the origin of the complex number plane and the phase is displayed as the angle versus positive real axis.
  • the received pilot sub carriers are multiplied with their nominal values '+1' or '-1' in order to bring each of them to a nominal value of '1' by removal of the modulation on the pilot sub carrier applied in the transmitter.
  • a mean pilot is determined by calculating the average magnitude of all received pilot sub carriers and calculating the average phase angle of all received pilot sub carriers. Both, the average magnitude and the average phase angle form the mean pilot by representing its position in the complex number plane.
  • these normalized pilot sub carriers are corrected for all possible time shifts within an expected tolerance that is up to 1/64 of the duration of a time symbol. All those possible time shifts are comprised in a set of different time shifts, wherein the different time shifts differ from each other by constant intervals.
  • a fourth step d for each time shift the standard deviation of the corrected pilot carriers from the mean pilot is calculated.
  • a fifth step e the time shift showing the minimum standard deviation for all pilot sub carriers from the mean pilot is determined.
  • a sixth step f the FFT output of all data and pilot sub carriers is corrected with the time shift corresponding to the minimum standard deviation.
  • step d) Because in real cases the time shift determined in step d) still can deviate from the optimum time shift by a fraction of the sampling interval, in an alternative additional seventh step g) (not shown), the pilot carriers that are treated according to the second step c) are rotated around the origin to have their mean value on the positive real axis. Thereby the fine correction for the symbol timing is obtained from the slope of the linear regression of the phase angle of these rotated pilots vs. sub carrier frequency. It is also thinkable to use the imaginary part of these pilot carriers vs. frequency.
  • step h) the FFT output of all data and pilot sub carriers that is already corrected with the time shift corresponding to the minimum standard deviation is again corrected using the fine tuning determined in step g).
  • Fig. 6 showing a tile 1 of a sub-channel.
  • the tile 1 comprises four columns C, each one belonging to sub carriers 2 having the same frequency f i and three rows R belonging to three succeeding time symbols 3 lying in succeeding time intervals t j .
  • the pilot sub carriers 4 are arranged in the corners of the tile 1.
  • the data sub carriers 5 are arranged between the pilot sub carriers 4.
  • the values of a received sub carriers differ from the values of the same sub carrier when transmitting it due to attenuation and phase shift, i.e. the values of the sub carriers at the receiver differ from the values the sub carriers had at the transmitter. Since the receiver knows the values X [i,j] of the transmitted pilot sub carriers 4 in advance, it is possible to calculate a channel response H [i,j] taking into account the values X [i,j] of the transmitted pilot sub carriers 4 and the values Y [i,j] of the received pilot sub carriers 4 according to:
  • the channel response H [i,j] is also called channel estimate. For each different time shift, the channel response H [i,j] of each pilot sub carrier 4 differs.
  • the time shift is chosen showing the minimum standard deviation for all pilot sub carrier channel estimates from the mean pilot sub carrier channel estimate. This time shift is applied to all data sub carriers belonging to the same sub-channel, or if more than one sub-channel is assigned to a SS, this time shift is applied to all data sub carriers belonging to the same SS, wherein the mean pilot channel estimate is calculated among all channel estimates of the pilot sub carriers belonging to the same SS.
  • sampling_interval 1 / N fft * N oversampling
  • delta_f 1 / N fft * N oversampling .
  • Each timing shift of delta_t sampling intervals causes a phase rotation of the n th sub carrier signal according to
  • phase n exp - i * 2 * pi * delta_t * f_subcarr n ,
  • n the sub carrier index that is defined by the position of the sub carrier in the frequency band.
  • the values, i.e. phase and magnitude of the received pilot sub carriers of one SS are considered.
  • all pilot sub carriers have their nominal values. If the received pilot sub carriers are multiplied with their nominal values, the resulting value of the magnitude of each pilot sub carrier is ideally '1'. If multi-path effects are present, the positions of the pilot sub carriers belonging to one SS, multiplied with their nominal value, are located on a curve as shown in Fig. 3, wherein an ideal timing is assumed.
  • the optimum time shift to be applied corresponds with the minimum of the standard deviation of the pilot carriers from their mean value. It has been proven by simulation, that the pilots of three sub-channels, i.e. 18 tiles are sufficient to find reliably a unique minimum even at very low signal-to-noise ratios (SNRs) below the operating threshold.
  • SNRs signal-to-noise ratios
  • the time shift still can deviate from the optimum time shift delta_t by a fraction of sampling_interval.
  • the pilot carriers treated according to Step b) have to be rotated around the origin to have their mean value on the positive real axis, wherein a fine correction for the timing is obtained from the slope of the linear regression of the phase angle of these rotated pilots vs. sub carrier frequency.
  • the time symbol of the OFDMA signal usually is enlarged by a CP to compensate multi-path effects in non line of sight conditions. Due to the CP, the demodulation can be performed even in presence of shifts in symbol timing, as long as they are within the length of the CP. In this case, the influence of the timing error is a phase rotation of each sub carrier proportional to its sub carrier index. IEEE 802.16e standard requires an allowed timing tolerance for each SS of 1/2 minimum CP, which means 1/64 of the symbol time duration. Other FFT sizes also are supported by this invention.
  • the invention is commercially applicable particularly in the field of production and operation of networks for wireless communication and data transmission.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
EP06300097A 2006-02-01 2006-02-01 Estmation de la synchronisation de symbole dans un système OFDMA Withdrawn EP1816818A1 (fr)

Priority Applications (4)

Application Number Priority Date Filing Date Title
EP06300097A EP1816818A1 (fr) 2006-02-01 2006-02-01 Estmation de la synchronisation de symbole dans un système OFDMA
KR1020060116759A KR20070079294A (ko) 2006-02-01 2006-11-24 Ｏｆｄｍａ 시스템에서의 심벌 타이밍 추정
US11/637,049 US20070177684A1 (en) 2006-02-01 2006-12-12 Symbol timing estimation in OFDMA systems
CNA2006101687929A CN101014030A (zh) 2006-02-01 2006-12-20 在ofdma系统中的符号定时估计

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
EP06300097A EP1816818A1 (fr) 2006-02-01 2006-02-01 Estmation de la synchronisation de symbole dans un système OFDMA

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EP1816818A1 true EP1816818A1 (fr) 2007-08-08

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EP (1) EP1816818A1 (fr)
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CN (1) CN101014030A (fr)

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US20080225792A1 (en) * 2007-03-12 2008-09-18 Qualcomm Incorporated Multiplexing of feedback channels in a wireless communication system
US7881392B2 (en) * 2007-03-30 2011-02-01 Hong Kong Applied Science And Technology Research Institute Co., Ltd. OFDM/OFDMA timing synchronization using non-consecutive pilot subcarrier assignment
US7830984B2 (en) * 2007-03-30 2010-11-09 Hong Kong Applied Science And Technology Research Institute Co., Ltd. OFDM/OFDMA channel estimation
KR101407045B1 (ko) * 2007-10-26 2014-06-12 삼성전자주식회사 파일롯 배치 방법, 기록 매체 및 전송 장치
US7965799B2 (en) * 2008-02-25 2011-06-21 Xilinx, Inc. Block boundary detection for a wireless communication system
US8238304B2 (en) * 2008-03-31 2012-08-07 Qualcomm Incorporated Apparatus and method for channel resource description
US20090268710A1 (en) * 2008-04-23 2009-10-29 Motorola, Inc. Uplink synchronization without periodic ranging in a communication system
CN103036833B (zh) * 2011-09-30 2017-10-13 锐迪科（重庆）微电子科技有限公司 一种ofdm系统定时同步控制方法及装置
CN103152294B (zh) * 2013-02-27 2016-08-31 北京晓程科技股份有限公司 基于信号消除进行噪声估计的方法及系统
US10620802B1 (en) * 2015-08-10 2020-04-14 Cadence Design Systems, Inc. Algorithmic modeling interface process

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US20070177684A1 (en) 2007-08-02
KR20070079294A (ko) 2007-08-06

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